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Make char::DecodeUtf16::size_hist more precise

New implementation takes into account contents of `self.buf` and rounds
lower bound up instead of down.
This commit is contained in:
Maybe Waffle 2022-01-27 00:25:17 +03:00
parent a7f375789b
commit cd4245d318

View file

@ -120,9 +120,21 @@ impl<I: Iterator<Item = u16>> Iterator for DecodeUtf16<I> {
#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let (low, high) = self.iter.size_hint();
// we could be entirely valid surrogates (2 elements per
// char), or entirely non-surrogates (1 element per char)
(low / 2, high)
// `self.buf` will never contain the first part of a surrogate,
// so the presence of `buf == Some(...)` always means +1
// on lower and upper bound.
let addition_from_buf = self.buf.is_some() as usize;
// `self.iter` could contain entirely valid surrogates (2 elements per
// char), or entirely non-surrogates (1 element per char).
//
// On odd lower bound, at least one element must stay unpaired
// (with other elements from `self.iter`), so we round up.
let low = low.div_ceil(2) + addition_from_buf;
let high = high.and_then(|h| h.checked_add(addition_from_buf));
(low, high)
}
}